1. Field of the Invention
The present invention relates to a friction plate used in a clutch or a brake of an automatic transmission (AT) of a vehicle, and, a wet-type multi-plate clutch having such a friction plate. More specifically, the present invention relates to an improvement in an oil passage formed in a friction surface of the friction plate.
2. Related Background Art
Generally, in a wet-type multi-plate clutch, friction plates and separator plates are alternately arranged between a drum and a hub of a clutch or a brake, so that engagement and disengagement of the clutch are performed by urging and releasing of a clutch piston.
Further, in recent years, request for reduction of fuel consumption in a vehicle has been increased more and more, and, in also an automatic transmission, in order to reduce power loss during disengagement of the clutch, reduction in drag torque between the friction plate and the separator plate has been requested.
Generally, in plate clutch used in the automatic transmission is many cases, the wet-type multi-designed so that lubricating oil can easily pass from an inner peripheral side to an outer peripheral side of the friction plate, thereby reducing the drag torque. An example of techniques for reducing the drag torque is known from, for example, Japanese Patent Application Laid-Open Nos. H11-141570 (1999) and 2005-76759. In clutches disclosed in the Japanese Patent Application Laid-Open Nos. H11-141570 (1999) and 2005-76759, the friction plate is provided with an oil groove having an inner peripheral side closed end for separation between the friction plate and the separator plate during the disengagement and an oil supplying passage extending in an inner-and-outer diameter direction for preventing seizure by supplying lubricating oil to a friction surface of the friction plate during the engagement.
However, in recent years, in order to enhance transmission response for enhancing a power performance as well as the reduction of the fuel consumption, a clearance between the friction plate and the separator plate has been made smaller in comparison with the conventional cases, with the result that, during an idle rotation, the drag torque tends to be increased due to the presence of the oil film therebetween.
The oil supplied to the oil passage extending from the inner diameter side to the outer diameter side is drawn to a friction material by a rotation and, if the drawn oil is brought between the friction plate and the separator plate, the oil is hardly discharged, and, particularly, in an area having small clearance between the friction plate and the separator plate and small number of revolutions, such a tendency conspicuous and the drag torque becomes greater due to viscosity between the friction material and the corresponding separator plate.
In conventional friction plates, since the lubricating oil on the friction surface is not discharged adequately, a request for further reducing the drag torque could not be satisfied. In particular, during a low rotation, since the oil interposed between the friction plate and the separator plate was not discharged adequately, the drag torque could not be reduced.
However, in order to meet a requirement for compactness and lighter-weight in recent automatic transmissions, it has been desired that a friction capacity of each friction plate be increased. To this end, the number of grooves to be formed in the friction plate, as well as the dimension of each groove, has been limited severely.